Electron beam (“e-beam”) lithography has successfully been employed in a variety of industrial applications to fabricate very small structures. An e-beam is focused on a target substrate to slowly and painstakingly ‘draw’, ‘carve’, or ablate a very fine pattern into the substrate. This procedure is repeated for each substrate required. E-beam lithography typically is capable of producing features having a dimension or resolution on the order of nanometers.
Though often effective, e-beam lithography is prohibitively slow and expensive for many applications, and is not readily applicable to mass-production. Techniques therefore have been developed to lower costs, decrease production times, and increase reproducibility. One such technique comprises using e-beam lithography to create a master, from which a stamp may be secondarily created. A stamping material (ink) is applied to the stamp, which is subsequently brought into contact with a surface. The stamping material is transferred to the surface at locations where the stamp contacts the surface. The surface may then be etched to remove surface material at all points that do not have stamping material, thereby replicating the stamp and selectively patterning the surface. Stamping of alkane thiols typically is capable of producing features having a dimension or resolution on the order of microns, though smaller structures are theoretically attainable.
Stamping of alkane thiols from a stamp onto a gold surface has been extensively investigated. The alkane thiol is absorbed either into or onto the stamp, and is then brought into contact with the gold substrate surface. Alkane thiols commonly consist of close-packed, independent chains that may be chemisorbed to a surface, and which often are used to modify surfaces, for example, to alter corrosion resistance and/or electrical properties, or to pattern the surfaces. Common alkane thiols include octadecanethiol and hexadecanethiol. These materials are typically applied from solution, e.g. ethanol or hexane, to surfaces such as gold, silver, or copper.
Although stamping of alkane thiols on gold surfaces has been extensively investigated, to date the method is still primarily a laboratory technique that has not been effectively transferred to industrial settings, due to the complexities of the stamping process. The simultaneous and often contradictory requirements of rapid diffusion and high solubility of the alkane thiol onto the stamp, appropriate mechanical characteristics of the stamp, fast reaction rates relative to surface diffusion rates of the alkane thiol onto the gold substrate, high irreversibility on the gold surface, and resistance of the stamping material to subsequent processing steps have been difficult to achieve. Thus, a central factor limiting adaptation of the laboratory technique to industrial applications has been the difficulties encountered while trying to achieve simultaneous control of multiple time-dependent, or rate, processes.
A newer surface patterning technique that has been developed to lower costs and decrease production times associated with e-beam lithography employs e-beam, UV, or x-ray resists. Such resists, and techniques for manufacturing them, are found, for example, in U.S. Pat. No. 4,717,645 to Kato et al.; U.S. Pat. No. 4,795,692 to Anderson et al.; and U.S. Pat. No. 4,868,241 to Hiscock et al.; all of which are incorporated herein by reference. A common resist technique comprises coating a substrate with a material that is sensitive to e-beam, UV, or x-ray radiation. The coating is selectively exposed to radiation, for example, with a focused electron beam that ‘traces’ the required pattern on the coating. Irradiation removes the coating at the point of exposure and provides a selectively patterned surface. This technique is similar to traditional e-beam lithography, except that the affected material comprises only a very thin, typically organic coating, thereby reducing the amount of material that is removed and the amount of time required to achieve patterning. The size of features attainable using resists depends on the energy source used for irradiation.
A significant drawback of resist techniques is that, although more rapid than traditional e-beam lithography techniques, time- and cost-intensive patterned irradiation of resists must still be conducted individually for each patterned surface. This drawback significantly limits the industrial viability of e-beam and x-ray resists.
Yet another technique that reduces the costs and production times associated with e-beam lithography is photolithography. Photolithography was developed prior to e-beam techniques, but provides many of the benefits of stamping and resist techniques. Photolithography typically requires production of a Master mask. The mask is placed over a substrate that has been coated with a photosensitive resist. A light source is shone through the patterned mask onto the resist, thereby patterning the surface. With a positive resist, material may be easily removed at all points on the surface that are exposed to irradiation. With a negative resist, material may be removed at all points not irradiated.
Although photolithography provides many of the benefits of e-beam lithography in a rapid and low cost procedure, the technique has fundamental limits. Specifically, photolithography typically cannot pattern surface structures having a size much smaller than the wavelength of the incident light. When using an i-line standard (365 nm UV light generated with mercury lamps) energy source, features on the order of about 500 nm are possible. Advanced focusing techniques may allow features slightly smaller than the wavelength of the incident light, for example, features as small as 300 nm with the i-line standard, but significantly smaller features are not possible.
Researchers have also examined the possibility of patterning with deep UV (“DUV”) light having a wavelength of 248 nm, generated with a krypton fluoride (“KrF”) excimer laser energy source 18. Furthermore, researchers have explored 193 nm laser sources 18, such as argon fluoride (“ArF”) excimer lasers. Researchers are still further exploring 157 nm laser sources 18, in the hopes of patterning surface features on the order of about 100 nm, when using advanced focusing techniques. However, systems using focusing techniques and operating at or below about 193 nm may suffer from degraded optics, since most lens materials, including fused silica or quartz, are absorptive at these wavelengths. Density variations in materials are also a problem at or below about 193 nm. Exotic alternative lens materials therefore are being examined, including, for example, calcium fluoride. Although calcium fluoride is highly transmissive, a significant drawback is that it is very difficult to fabricate. Additionally, if extreme UV (13 nm) or X-ray (<3 nm) are light sources ever considered for mass-production purposes, such as in the production of microelectronics, it is expected that complex and cost-intensive new lasers or synchrotron systems will be required to generate adequate extreme UV or X-ray photons to meet production requirements.
Especially in the field of microelectronics, the drive for smaller and smaller structures is rapidly creating a need to pattern surface structures smaller than those possible today with standard photolithography employing i-line standard UV light. In many cases, traditional e-beam techniques are the only practical recourse for providing such fine structures.
In view of the drawbacks associated with prior art patterning techniques, it would be desirable to provide methods and apparatus for patterning surfaces that overcome these drawbacks.
It would be desirable to provide methods and apparatus that reduce costs and production times, as compared to e-beam techniques.
It also would be desirable to provide methods and apparatus for patterning surfaces that require control of fewer rate processes.
It would be desirable to provide methods and apparatus for patterning surfaces that may be replicated using a stamping or masking technique.
It would be desirable to provide methods and apparatus that theoretically enable patterning of surface structures having a size smaller than achievable with standard photolithography techniques.
It would be desirable to provide methods and apparatus that are applicable to industrial applications.